Method of joining carbides to base metals



ite States] Atomic Energy Commission No Drawing. Filed Nov. 12, 1959, Ser. No. 852,585

' 4 Claims. (Cl. 29-420) Thepresent invention relates to methods for joining refractory compounds to metallic bases, and in particular deals with the joining of metallic carbides to such bases.

In the arts dealing with nuclear rockets, thermoelectric transducers and vacuum tubes utilizing carbide emitters, it has frequently become necessary to join a refractory carbide to a solid metallic base. Examples of such joints appear in the co-pending application of Grover et al., S.N. 821,339, wherein are described structures utilizing emitter electrodes of UC, ZrC and solid solutions of UC and ZrC. These electrodes are joined to a base of tantalum or niobium, and it is the primary purpose of the present invention to furnish methods whereby such joints are made.

It is known that other persons have attempted to join UC to a tantalum base by either melting the UC directly on the tantalum or by brazing the two materials together with a refractory alloy such as one of titanium and niobium having a melting point of about 2200 C. Such attempts were highly unsatisfactory, as uranium is liberated in the melting technique and extremely brittle joints .are obtained in the attempts at brazing.

The present inventors have conceived and reduced to pnactice a technique for achieving strong solid joints between such refractory carbides and refractory base metals which avoids the ditliculties experienced by prior workers in the art in liberating uranium, such joints having none of the brittleness characteristic of the prior art brazed joints. The methods of the present inventors are based on sound thermodynamic and kinematic considerations, and should be applicable to all cases where it is sought to join a refractory metal compound to a refractory base metal, including in such refractory metal compounds not only carbides but nitrides, borides and silicides.

in general, the technique of the present inventors involves first of all the consideration of whether or not the base metal forms carbides which are thermodynamically more stable than the carbide which is to be joined to it. If the carbide of the base metal is the more stable, and additional step is necessary before the joint can be made. If this step is not taken, the base metal will replace the metal in the carbide to be joined and will liberate such metal. Thus tantalum and niobium displace uranium from UC. While the reaction proceeds rather slowly at low temperatures, it is fast enough to prevent the obtaining of sound joints at the temperatures of joining and will proceed even more rapidly if the joint is exposed to temperatures exceeding 1800 C. in the course of operation. a

If the base metal forms carbides which are less stable than the carbides to be joined to the base, the method of the present inventors may or may not be necessary, depending on the solubility of such base'metal in the carbide to which it is joined. Thus TaC is less stable than ZrC and the tantalum will not displacezirconium from ZrC. However, ZrC and tantalum form a low melting eutectic, having a melting point of 2500x200 C. While the joint can be satisfactorily made, it would dissolve at the operating temperatures employed in such devices as thermoelectric transducers. For such joints atent ii 3,020,632 Patented Feb. 13, 196 2 2 in such applications, the method of the present inventors is essential, but it may be omitted in those cases where the base metal and carbide joined to it form no such low melting eutectic.

The second consideration is Whether or not the base metal forms carbides which are sufliciently soluble in the carbide to be joined that a solid solution may readily be formed. Thus it is known that TaC and NbC form acontinuous series of solid solutions with UC, ZrC, and solid solutions of UC and ZrC. For such base metal carbides, it is a simple matter to form a solid solution joint with the other carbides mentioned. There are known instances, however, of base metals whose carbides have limited solubilities in UC or ZrC, and joints therebetween are much more dilficult to achieve.

The third consideration is the ease with which a carbide layer may be formed on the base metal, and the tenacity of such layer to the base metal. Thus itis known that taut-alum and niobium may be readily carburized by heating them in contact with graphite powder in a vacuum or inert atmosphere. The resulting carbide surfaces are extremely thin and tenacious, and are apparently impervious to migration of the underlying base metal. In the case of tantalum, there is an inner layer adjacent to the tantalum consisting of Ta C. The outer f layer, usually thinner, is a TaC phase. Suchcarburizing may also be accomplished by heating .thebase metal in gas mixtures containing hydrocarbons. v V V 'The method of the present invention'consists essentially of heating the out-gassed metal base in contact with out-gassed graphite powder to a temperature of about 2000-2200 C. for a period of about .15 minutes in a non-oxidizing atmosphere (vacuum or inert gas) and UC, or the carbide to be joined may be sintered to the carburized base at a somewhat lower temperature. The latter alternative may be employed in any event, and may be a sintering of an already compacted carbide to the carbide surface of the base metal or may be a simultaneous compacting of a mass of carbide powder and a sintering of such compact to the carburized base metal. It has not been found necessary to use pressure together with the high temperature treatment, although the process is shortened by the application of pressure. A refinement which reduces the necessary sintering temperature somewhat includes the use of sintering aids such as iron, cobalt, or nickel in the carbide to be joined. Such a sintering aid may of course be left in place in the final product, in those instances where it does no harm, or it may be removed by heating to 2000-2200 C. in vacuum. It has been found that less than about 5 weight percent of the sintering aid is all that can be profitably used, and that larger amounts are likely to promote crack formation in such carbides as UC.

A specific example of the present invention is furnished to enable those skilled in the art to practice the same. This example depicts the preparation of a fuel pin used in the aforementioned thermoelectric generator of Grover et al. and described in their aforementioned application.

Example (2) The solid solution was then powdered in a diamend mortar and pestle to to +200 mesh. The powdering was done in an inert atmosphere dry box.

(3) A calculated amount of the solid solution was I and heating begun.

weighed out, and or less by weight nickel was added as both a binder and sintering aid.

(4) The powder was transferred to a graphite die, assembled with the tantalum collar in placeand placed in a vacuum line.

(5) An eddy current concentrator was used to heat the die, powder and tantalum collar. I I

(6) The system was evacuated to about mm. Hg The temperature was 2000 C. i200 C. and the pressure 4000-6000 psi. Both the temperature and pressure were maintained until the pres? sure dropped extremely slowly on release of the press handle.

(7) After cooling, the. pressed piece was removed'from the die and cleaned.

(8) The binder and sintering aid was then removed by heating from 1 800-2200 C. in vacuum.

The resulting cooled joint was 'very strong and tough,

with'the base metal distinctly separated from the can hides joined thereto and with no liberation of the metals in the carbides. nickel powder with the carbide mixture is not essential, as similar joints have been made at a slightly higher temperature without such a binder; The pressurizing step is also unnecessary, but it does shorten the time neces sary to form the joint. Similarly formed strong joints As previously noted, the use of the selected from the class consisting of uranium carbide and zirconium carbide to a surface of, a refractory metal base selected from the class consisting of tantalum and niobium comprising carburizing the surface of said. metal base and 'sinteringsaid mass and said base at a tempera ture in the rangeof 1700? C.-2200 C. in a non-oxidizing atmosphere while said base is held in contact with said carbide during the sintering step.

2. A method of joining a consolidated mass composed of at least one refractory metal carbide in powder form selected from the'class consisting of uranium carbide and percent selected from the class consisting of iron nickel, y and cobalt, and said method includes the finalstep of removing such sinteringaid by heating in a vacuum.

4. The method of claim 2 in which said pressing is about 5000 pounds per square inch.

References Cited in the file of this patent V UNITED STATES PATENTS 2,525,565 Sorg ,et al. Oct. 10, 1950 2,652,621

Nelson Sept; 22, 1953 

1. A METHOD OF JOINING A CONSOLIDATED MASS COMPOSED OF AT LEAST ONE REFRACTORY METAL CARBIDE IN POWDER FORM, SELECTED FROM THE CALSS CONSISTING OF URANIUM CARBIDE AND ZIRCONIUM CARBIDE TO A SURFACE OF A REFRACTORY METAL BASED SELECTED FROM THE CLASS CONSISTING OF TANTALUM AND NIOBIUM COMPRISING CARBURIZING THE SURFACE OF SAID METAL BASE AND SINTERING SAID MASS AND SAID BASE AT A TEMPERATURE IN THE RANGE OF 1700* C.-2200* C. IN A NON-OXIDIZING ATMOSPHERE WHILE SAID BASE IS HELD IN CONTACT WITH SAID CARBIDE DURING THE SINTERING STEP. 